To many civil libertarians, the conviction of Tarek Mehanna for aiding Al Qaeda dangerously undermined free speech rights and delivered a stark message that holding radical ideas can be punished under the guise of national security.

To terrorism specialists, the jury’s swift guilty verdict validated the government’s efforts to prosecute domestic terror suspects and sharpened the line between protected speech and active support for terrorism.

The polarized reactions to the high-profile case mirrored the contrasting arguments in federal court, where the Boston-area extremist was accused of being a terrorist sympathizer bent on waging jihad against American soldiers, and defended as a devoted Muslim who never directly worked with terrorists or acted on his extreme beliefs.

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Carol Rose, director of the American Civil Liberties Union of Massachusetts, said the charges against Mehanna, a 29-year-old who grew up in Sudbury, criminalized what should be considered protected political speech, such as translating Arabic texts into English and searching for information about the Sept. 11 attackers.

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“While it may not be popular, in America it’s never been a crime,’’ she said. “This raises serious First Amendment concerns.’’

The defense argued that Mehanna never worked in coordination with any terrorist group, or at one’s direction, and noted that “independent advocacy’’ of a cause is protected as free speech.

“This case can’t help but send a message to people that if you support an unpopular cause, you’d better be looking over your shoulder because the federal government may be right behind you,’’ Mehanna’s lawyer, J.W. Carney Jr., said after the verdict. Carney said he would appeal the verdict, which carries a possible sentence of life in prison.

Karen Greenberg, director of the Center on National Security at Fordham Law School, said the verdict shows that the US legal system is inherently biased against terrorism suspects because of laws that broadly define what constitutes support to terrorist groups. Juries have consistently sided with prosecutors.

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“We still refuse to tolerate the notion that there can be an acquittal in a terrorism case,’’ she said. “We’re not yet at a place where we can make the distinction between the aspirational and operational. It’s just not a possibility.’’

Greenberg noted that the prosecution never proved that Mehanna attended a terrorist training camp.

Greenberg, a specialist in terrorism and civil liberties who closely followed the trial, said Congress has broadened statutes that make it a crime to give “material support’’ to terrorist groups since the Sept. 11 attacks, and prosecutors have used those laws more aggressively.

Mehanna was convicted of conspiracy to provide material support or resources to a foreign terror organization, conspiracy to provide material support to terrorists, providing and attempting to provide material support to terrorists, conspiracy to kill in a foreign country, conspiracy to lie to federal investigators, and two counts of lying to federal investigators.

George Brown, a Boston College law professor, said the lack of evidence linking Mehanna to terrorist groups showed how far the government will go to prosecute suspects in hopes of preventing attacks.

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“It’s a tremendous win for the preventive paradigm,’’ Brown said. “You don’t even wait for an attempt at a crime before going after potential terrorists.’’

‘We still refuse to tolerate the notion that there can be an acquittal in a terrorism case.’

The verdict has far-reaching implications, Brown and others said, because it shows that aggressive prosecution of antiterrorism statutes can lead to convictions, even when evidence of direct interaction is thin.

“It’s a vindication of that strategy,’’ he said, saying there was “simply no coordination’’ between Mehanna and Al Qaeda.

In his instructions to the jury last week, US District Judge George O’Toole Jr. said that to find Mehanna guilty of supporting terrorism, jurors had to conclude he worked in coordination with, or at the direction of, a terrorist organization.

Prosecutors said Mehanna translated and disseminated jihadist materials and videos on the Internet in a deliberate campaign to support Al Qaeda. A search of his computer found countless documents promoting Al Qaeda, FBI agents testified.

Prosecutors also focused on Mehanna’s visit to Yemen in 2004, saying he was seeking terrorism training even though he failed to find a camp and returned.

Several legal specialists speculated that the Yemen visit was pivotal to the jury’s decision.

Glenn Sulmasy, professor of law at the US Coast Guard Academy and author of “The National Security Court System: A Natural Evolution of Justice in an Age of Terror,’’ called the evidence against Mehanna substantial.

“This went well beyond political statements or someone rallying to a cause,’’ he said. “If you take the totality of the circumstances, it was someone trying his best to engage in jihad overseas and advocating to take up arms against America.’’

Sulmasy said the conviction showed the Obama administration’s commitment to prosecuting domestic terrorist suspects. He said it would serve as a deterrent.

“This sends a signal that we’re not going to tolerate this kind of activity that is trying to skirt the lines of the First Amendment,’’ he said. “The government has to be able to prevent attacks before they occur.’’

Specialists said the antiterrorism statutes give the government considerable leeway in convicting suspects, often to a worrisome extent.

“The First Amendment defense has not been very successful in these cases,’’ said Ragini Shah, a professor at Suffolk University Law School. “Often, the government doesn’t really have to prove you did much of anything.’’